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Microprocessor and Microcontroller Fundamentals
PIC18F4520
Harsh Shrimal
IGS Labs Technology
Microcontrollers
Embedded Systems Operations managed behind the scenes by a
microcontroller
Microcontroller (MCU) An integrated electronic computing device that
includes three major components on a single chipMicroprocessor (MPU)Memory I/O (Input/Output) ports
2
Microcontrollers
Support Devices Timers A/D converter Serial I/O
All components connected by common communication lines called the system bus.
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Block Diagram
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Microprocessor (MPU)
A group of electronic circuits fabricated on a semiconductor chip that can read binary instructions written in memory and process binary data according to those instructions
CPU and MPU
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Memory
A semiconductor storage device consisting of registers that store binary bits
Two major categories Read/Write Memory
(R/W) Read-only-Memory
(ROM)
D7 D0
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Input/Output (I/O)
Input devices such as switches and keyboards provide binary information to the microprocessor
Output devices such as LEDs, video screens, and printers receive information from the microprocessor
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Microprocessor-Based Systems
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Microprocessor Architecture
The MPU communicates with memory and I/O using the system bus consisting of: Address bus: unidirectional and carries memory
and I/O addresses Data bus: bidirectional; transfers binary data and
instructions between MPU and memory and I/O Control lines: Read and Write timing signals
asserted by MPU
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Microprocessor-Based System with Buses: Address, Data, and Control
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Data Format (8-bit)
Unsigned Integers: All eight bits (Bit7 to Bit0) represent the magnitude of a number Range 00 to FF in Hex and 0 to 255 in decimal
Signed Integers: Seven bits (Bit6 to Bit0) represent the magnitude of a number. The eighth bit (Bit7) represents the sign of a number.
The number is positive when Bit7 is zero and negative when Bit7 is one.
Positive numbers: 00 to 7F (0 to 127) Negative numbers: 80 to FF (-1 to -128) All negative numbers are represented in 2’s complement
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Data Format (8-bit)
Binary Coded Decimal Numbers (BCD) 8 bits of a number divided into groups of four,
and each group represents a decimal digit from 0 to 9
Four-bit combinations from A through F in Hex are invalid in BCD numbersExample: 0010 0101 represents the binary coding of
the decimal number 25 which is different in value from 25H.
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PIC18F Microcontroller Families
PIC microcontrollers are designed using the Harvard Architecture which includes: Microprocessor unit (MPU) Program memory for instructions Data memory for data I/O ports Support devices such as timers
PIC18F – MPU and Memory
Microprocessor Unit
Arithmetic Logic Unit (ALU) WREG – working register (8-bit accumulator) Status register that stores flags (5-bits) Instruction decoder (16-bit instructions)
Registers BSR (4-bit register used in direct addressing the data memory)
FSR (12-bit registers used as memory pointers in indirect addressing data memory)
Program Counter (PC) (21-bit register that holds the program memory address while executing programs)
PIC18F - Buses
Address bus 21-bit address bus for program memory addressing
capacity: 2 MB of memory 12-bit address bus for data memory addressing capacity:
4 KB of memory
Data bus 16-bit instruction/data bus for program memory 8-bit data bus for data memory
Control signals Read and Write
PIC18F452/4520 Memory
Program Memory: 32 K Address range: 000000 to 007FFFH
Data Memory: 4 K Address range: 000 to FFFH
PIC18F452—I/O Ports A and B
MCU Support Devices
PIC18F Special Features
Sleep mode Watchdog timer (WDT) Code protection In-circuit serial programming In-circuit debugger
Assembly code:
1 Problem statement: Write instructions to light up alternate LEDs at
PORTC
000000 0E00 MOVLW 00 ;Load W with 0s000002 6E94 MOVWF TRISC,0 ;Set PORTC as output000004 0E55 MOVLW 0x55 ;Load 55 to turn on
LEDs000006 6E82 MOVWF PORTC,0 ;Turn on LEDs000008 0003 SLEEP ;Power Down
Illustration
Execution of the instruction:
MOVWF PORTC
ASSEMBLY LANGUAGE
PIC18F
Addressing Modes
Method of specifying of an operand Immediate (Literal) addressing
The operand is a number that follows the opcode
MOVLW 0X10 Direct addressing
The address of the operand is a part of the instruction
MOVWF 0X20 Indirect addressing
An address is specified in a register (pointer) and the MPU looks up the address in that register
Instruction Format
The PIC18F instruction format divided into four groups Byte-Oriented operations Bit-Oriented operations Literal operations Branch operations
Label Opcode Operand Comment
START: MOVLW 0xF2 ;Load F2H in W
↑ ↑ ↑ ↑
Tab Tab Tab Semicolon
1. Data Transfer Instructions.:
MOVLW k ; Move constant to W
MOVLW 0X20MOVWF f ; Move W to f
MOVWF 0X20
MOVFF F,F ; Move f to f
MOVFF 0X20,0X30
2. Arithmetic Instructions.:
ADDLW K ;Add KH to W ADDWF f, d ;Add W and f W +f -> d SUBLW k ;Subtract W from constant SUBWF f,d ;Subtract W from f INCF F DECF F CLRF F SETF F
ADD instruction
LIST P=18F4520 ;directive to define processor#include <P18F4520.INC>;processor specific variable definitions
REG0 EQU 0X20REG1 EQU 0X30REG2 EQU 0X40ORG 00GOTO MAINMAINMOVLW 0x0FMOVWF REG0MOVLW 0x20MOVWF REG1ADDWF REG0,WMOVWF REG2SLEEP
3. Logic Instruction.:
ANDLW k ;Logical AND with W with K IORLW k ; Logical OR with W with constant XORLW k ; Logical exclusive OR with W with f ANDWF f, d ; Logical AND with W with f IORWF f, d ; Logical OR with W with f XORWF f, d ; Logical exclusive OR with W with
constant
4. Bit oriented instruction.:
BSF file reg, bit ; bit set file reg.
BCF file reg, bit ; bit clear file reg.
BTG file reg, bit ; bit toggle file reg.
BTFSS file reg, bit ; bit test file reg. skip next instruction if bit is set
BTFSC file reg, bit ; bit test file reg. skip next instruction if bit is clear
5. Program Control Instructions.:
BTFSC f ,b BTFSS f ,b DECFSZ f ,d INCFSZ f ,d GOTO k CALL k RETURN ;Return from subroutine RETLW k ;Return with constant in W RETFIE ;Return from interrupt
6. Other instructions.:
NOP ; No operation CLRWDT ; Clear watchdog timer SLEEP ; Go into sleep mode
1. Initializing a File Register
Problem on port:
2. Loop statements.:
INCFSZ f, dIncrement the contents of register f. Store the result in W (if d = 0) or in register f (if d =1). If the incremented value of f = 0, skip the next line of code.
DECFSZ f, dDecrement the contents of register f. Store the result in W (if d = 0) or in register f (if d =1). If the decremented value of f = 0, skip the next line of code.
BTFSC f, bTest bit b of register f. If bit b of register f is clear (that is, it equals zero), skip the next line of code.
BTFSS f, bTest bit b of register f. If bit b of register f is set (that is, it equals one), skip the next line of cod
Loop Example
COUNT1 EQU 0x01 ;Counter is REG01ORG 00GOTO START
ORG 0x20START: MOVLW 0x05 ;Initialize Counter to 5
MOVWF COUNT1LOOP1: DECFSZ COUNT1;Decrement Counter, Skip if 0
GOTO LOOP1SLEEP ;DoneEND
Nested Loop.:
Data on Port C.:
Special Function Registers
Data registers associated with I/O ports, support devices, and processes of data transfer
I/O Ports (A to E) InterruptsEEPROMSerial I/OTimersCapture/Compare/PWM (CCP)Analog-to-Digital (A/D) Converter
File register (data RAM):
The file register data RAM in PIC is divided into two sections:
• Special Function Registers (SFR)
• General-Purpose Registers (GPR)
SFRs (Special Function Registers)
The Special Function Registers (SFRs) are dedicated to specific functions such as ALU status, timers, serial communication, I/O ports, ADC, and so on. The function of each SFR is fixed by the CPU designer at the time of design because it is used for control of the microcontroller or peripheral. The PIC SFRs are 8-bit register.
P18F4520
Timers
Timer0 Timer1 Timer2 Timer3
Timer0.:
Software selectable operation as a timer or counter in both 8-bit or 16-bit modes
Readable and writable registers Dedicated 8-bit, software programmable
prescaler Selectable clock source (internal or external) Edge select for external clock Interrupt-on-overflow
Registers associated with Timer0.:
Prescaler.:
By default the timer is incremented by one in every instruction cycle.
Say that we are using the crystal of 4MHz i.e. timer will increment at instruction cycle rate of
1MHz. So TMR0 will increment in every 1us. So maximum we can make a delay of 255us. To increase this period we use prescaler.
Let PS<2:0>=111 Now TMR0 will increase in 256 instruction cycle. i.e. We can produce the delay of
255 * 255us= 65.28ms
Period.:
Equation to calculate TMR0L value for period.:
Period = (256 - TMR0)*(4/fosc)*(Prescaler)
For example if Period = 1 ms and fosc = 4 MHz and using
Prescaler 1:41ms = (256 - TMR0)(1us)(1)TMR0 = 6 = 0x06
Timer1.:
Software selectable operation as a 16-bit timer or counter
Readable and writable 8-bit registers (TMR1H and TMR1L)
Selectable clock source (internal or external) Interrupt-on-overflow Reset on CCP Special Event Trigger Device clock status flag (T1RUN)
Registers associated with Timer1.:
Timer2.:
8-bit timer and period registers (TMR2 and PR2, respectively)
Readable and writable (both registers) Software programmable prescaler (1:1, 1:4 and
1:16) Software programmable postscaler (1:1 through
1:16) Interrupt on TMR2-to-PR2 match Optional use as the shift clock for the MSSP or
PWM module
Registers associated with Timer2.:
Timer2 Operation.:
In normal operation, TMR2 is incremented from 00h on each clock (FOSC/4).
The value of TMR2 is compared to that of the period register, PR2 on each clock cycle.
When the two values match, the comparator generates a match signal as the timer output.
This signal also resets the value of TMR2 to 00h on the next cycle and drives the output counter/ postscaler.
T2CON.:
Timer3.:
Software selectable operation as a 16-bit timer or counter
Readable and writable 8-bit registers (TMR3H and TMR3L)
Selectable clock source (internal or external) Interrupt-on-overflow Module Reset on CCP Special Event Trigger
Timer3.:
The Watchdog Timer.:
Suppose you have written a program that is continuously running on a PIC.
Now consider this two situations.:1. Let us say that the PIC is monitoring an
input. When this input goes high, it jumps to another part of the program. What happen when its not getting high?
2. The another situation after a long period of time, the program gets stuck somewhere and the PIC gets caught in a loop.
So what is a Watchdog Timer?
What’s needed in both cases is some kind of reset if the program gets stuck.
This is the purpose of a watchdog circuit.
How WDT works?
Well, inside the PIC there is a resistor/capacitor network.
This provides a unique clock, which is independent of any external clock that you provide in your circuit.
Now, when the Watchdog Timer is enabled, a counter starts at 00 and increments by 1 until it reaches FF.
The only way we can stop the WDT from resetting the PIC is to periodically reset the WDT back to 00 throughout our program.
Registers associated with WDT.:
P18F4520
INTERRUPTS
Interrupts.:
Interrupts are situations that the CPU can't predict when they will happen, they can happen any time, so the CPU does not wait for them. So the CPU keeps on doing its normal job unless and interrupt occurs.
External interrupts TIMER interrupts Analog to Digital Converter Interrupts Data Interrupts
Interrupts.:
The PIC18F devices have multiple interrupt sources.:
High priority - Vector at 0008h
Low priority - Vector at 0018h
High priority interrupt events will interrupt any low priority interrupts that may be in progress.
Registers associated with interrupts.:
RCON INTCON INTCON2 INTCON3 PIR1, PIR2 PIE1, PIE2 IPR1, IPR2
Flag bit Enable bit Priority bit
Timer0 interrupt
Timer0 interrupt
Analog to Digital Converter (ADC).:
Many electrical signals around us are Analog in nature. That means a quantity varies directly with some other quantity.
The first quantity is mostly voltage while that second quantity can be anything like temperature, pressure, light, force or acceleration.
For example in LM35 temperature sensor the output voltage varies according to the temperature, so if we could measure voltage, we can measure temperature.
ADC.:
But most of our computer (or Microcontrollers) are digital in nature.
They can only differentiate between HIGH or LOW level on input pins. For example:
If input is more than 2.5v it will be read as 1 and if it is below 2.5 then it will be read as 0 .
So we cannot measure voltage directly from MCUs. To solve this problem most modern MCUs have an ADC
unit. In PIC18F4520 there are 13 analog input channels, they are
named AN0, AN1 etc.
Specifications of ADCs.:
Most important specification of ADCs is the resolution. This specifies how accurately the ADC measures the analog input signals.
Common ADCs are 8 bit, 10 bit and 12 bit. For example: If the reference voltage of ADC is 0 to 5v then
1. A 8-bit ADC will break it in 256 divisions so it can measure it accurately up to 5/256 v= 19mV approx.
2. While the 10-bit ADC will break the range in 5/1024 = 4.8mV approx.
So you can see that the 8-bit ADC can't tell the difference between 1mV and 18mV. The ADC in PIC18 are 10-bit.
Reference Voltage.:
The reference voltage specifies the minimum and maximum voltage range of analog input. In PIC 18 there are two reference voltage, one is the Vref- and one is Vref+.
The Vref+ and Vref- pins are available in PIN5 and PIN4 of the PIC18F4520 chip.
Acquisition Time.:
When an specific channel is selected the voltage from that input channel is stored in an internal holding capacitor.
It takes some time for the capacitor to get fully charged and become equal to the applied voltage.
This time is called acquisition time. A safe value is 2.45uS, so acquisition time
must be set to any value more than this.
ADC Clock (Conversion time).:
ADC Requires a clock source to do its conversion, this is called ADC Clock. The time period of the ADC Clock is called TAD.
There are Seven possible option.1. 2 x TOSC2. 4 x TOSC3. 8 x TOSC4. 16 x TOSC5. 32 x TOSC6. 64 x TOSC7. Internal RC
PIC18 ADC.:
The Analog-to-Digital (A/D) converter module has 10 inputs for the 28-pin devices and 13 for the 40/44-pin devices.
This module allows conversion of an analog input signal to a corresponding 10-bit digital number.
The module has five registers:1. A/D Result High Register (ADRESH)2. A/D Result Low Register (ADRESL)3. A/D Control Register 0 (ADCON0)4. A/D Control Register 1 (ADCON1)5. A/D Control Register 2 (ADCON2)
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ADC BLOCK DIAGRAM.:
Serial
Communication
Types of Serial Communication.:
Synchronous (I2C) Asynchronous (UART) In USART a start bit and stop bits are used to
synchronize the incoming data the.
Serial Communication.:
What a level converter:
RS232 level signals (HIGH=-12V LOW=+12V) from PC
TTL level signal (HIGH=+5V LOW=0V) to be fed to MCU
Synchronization.:
Baud Rate.:
As there is no "clock" line so for synchronization accurate timing is required so transmissions are carried out with certain standard speeds.
The speeds are measured in bits per second. The speed is also known as baud rate.
Some standard baud rates are1200 192002400 384004800 576009600 115200... etc
Transmitting device.:
For example we chose the speed as 9600bps(bits per second).
As we are sending 9600 bits per second one bits takes 1/9600 seconds or 0.000104 sec or 104 uS
then it send each bits with duration = 104uS
Receiving device.:
ENHANCED UNIVERSAL SYNCHRONOUS RECEIVER TRANSMITTER (EUSART).:
Also known as a Serial Communications Interface or SCI.
The EUSART can be configured as a full-duplex asynchronous system that can communicate with peripheral devices, such as
CRT terminals personal computers
It can also be configured as a half-duplex synchronous system that can communicate with peripheral devices, such as
A/D or D/A integrated circuits serial EEPROMs, etc.
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The pins of the Enhanced USART are multiplexed with PORTC. In order to configure RC6/TX/CK and RC7/RX/DT as a USART:
bit SPEN (RCSTA<7>) must be set (= 1) bit TRISC<7> must be set (= 1) bit TRISC<6> must be set (= 1)
Registers.: Transmit Status and Control (TXSTA) Receive Status and Control (RCSTA) Baud Rate Control (BAUDCON)
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CAPTURE/COMPARE/PWM(CCP) MODULES.:
A capture, compare, and PWM, module, or CCP for short, is designed into the PicMicro to assist with measurement or control of time based pulse signals, module contains:-
a 16-bit register which can operate as a Capture register
a 16-bit Compare register a 10-bit PWM Master/Slave Duty Cycle
register.
CCP Module Configuration.:
Each Capture/Compare/PWM module is associated with a control register (generically, CCPxCON) and a data register (CCPRx).
The data register, in turn, is comprised of two 8-bit registers: CCPRxL (low byte) and CCPRxH (high byte).
All registers are both readable and writable.
CCP MODE – TIMER RESOURCE.:
Capture Mode.:
In Capture mode, the CCPRxH:CCPRxL register pair captures the 16-bit value of the TMR1 or TMR3 registers when an event occurs on the corresponding CCPx pin.
An event is defined as one of the following:• every falling edge• every rising edge• every 4th rising edge• every 16th rising edge
The event is selected by the mode select bits, CCPxM3:CCPxM0 (CCPxCON<3:0>).
When a capture is made, the interrupt request flag bit, CCPxIF, is set; it must be cleared in software.
Compare Mode.:
In Compare mode, the 16-bit CCPRx register value is constantly compared against either the TMR1 or TMR3 register pair value. When a match occurs, the CCPx pin can be:
• driven high
• driven low
• toggled (high-to-low or low-to-high)
• remain unchanged (that is, reflects the state of the I/O latch) The action on the pin is based on the value of the mode select
bits (CCPxM3:CCPxM0). At the same time, the interrupt flag bit, CCPxIF, is set.
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PWM Mode.:
In Pulse Width Modulation (PWM) mode, the CCPx pin produces up to a 10-bit resolution PWM output.
Since the CCP2 pin is multiplexed with a PORTB or PORTC data latch, the appropriate TRIS bit must be cleared to make the CCP2 pin an output.
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